Many machines or devices include stems, variously referred as pins, shafts, spindles, etc., extending from a fluid-containing member to transmit power into or out of it. All these stems or spindles need sealing to prevent the fluid from leaking around the stem, but some stems dispose one seal either at the stem cylinder or at the stem shoulder, and some stems dispose both the two.
For example, valves, as the controlling unit for fluid conveying, have a valving member, such as the ball in ball valves, the gate in gate valves, etc. The valving member is interposed in a flow path, and has an open position, which allows medium to flow through the valve, and a closed position, which prevents medium from flowing through the valve. The shifting of the valving member between the two positions is realized by a stem extending out of the valve. The stem needs sealing to prevent medium from leaking around the stem out of the valve into atmosphere. However, some valves need only one seal either for stem cylinders or for stem shoulders, and other valves need both the two.
Stems, such as stems for ball valves or gate valves, shall not be ejected through the pressure boundary by internal pressure when the stem packing and/or retainer have been removed in accordance with the related standards, so that the stem shall have a stop shoulder to be pressed against the inside of stem exits to prevent the stem from being removed through the stem exit opening at the bottom of the stuffing box of valves. That is to say, the stem, particularly the stem of ball valves, has two seals, one stem shoulder seal located at the inside of the stem exit and one stem cylinder seal starting at the outside of the stem exit and extending through the stuffing box. When assembled, the stuffing around the stem is compressed against the bottom of the stuffing box starting at the outside of the stem exit by glands, Belleville washers and the nut engaged by thread with the stem to provide a cylinder seal for the stem, and at the same time the stop shoulder of the stem is pulled tight against the inside of the stem exit opposite the bottom of the stuffing box to provide a shoulder seal for the stem.
The conventional metal stem stop shoulder is a plain (see FIG. 3a) or taper (see FIG. 3b) flange on stems, and needs a plain or conical gasket between the stop shoulder and its thrust step at the inside of stem exits, which provides a metal-to-non-metal stem shoulder soft seal. Patent U.S. Pat. No. 6,129,336 also disclosed a metal-to-non-metal stem shoulder soft seal using a ball shoulder as the stem stop shoulder and a spherical gasket as the thrust gasket between the stop shoulder and its thrust step. A leak will happen to the plain and the conical assemblies and not happen to the spherical assembly which may turn about the sphere center when the stem inclines under operational and non-operational loads.
The conventional packing as stem cylinder seals is a group of either plain rings (see FIG. 3a) or V-rings (see FIG. 3b). These grouped packing rings rely on axial compressing forces to accomplish at the same time the stem cylinder seal, preventing medium from leaking along stem, and the box seal, preventing medium from leaking along box wall. The box seal is a static seal easy to be accomplished, and the stem cylinder seal is often a dynamic stem seal difficult to be accomplished. Consequently, realizing the stem cylinder seal is naturally critical for the accomplishment of stem seals, and inherently needs a radial compression of packing for providing a radially sealing stress. However, the packing can only be compressed by an originally axial force. Thus it is critical for realizing the stem cylinder seal to enable the packing to obtain a radially compressing component from an originally axially compressing force or a radial movement from an originally axially compressing movement.
It is well known that any packing material in stuffing boxes, if its supporting and its compressing planes both are square to stem axis, can have a radial movement only when it is axially compressed to yield and deform, whereas, if both are a cone not friction-locked for the radial movement, only the local material in contact with the conical surfaces may radially be compressed to move because forces are axially decreasedly transmitted in packing material with frictional resistance. That is to say, there are altogether two ways to enable the packing to yield an efficient radial sealing movement; one is to enable the whole packing to fully yield and deform to get some packing material a bi-radial movement, and the other is to enable the whole packing to suitably radially shrink to get the packing material a uni-radial movement. To accomplish a stem cylinder seal, the packing in bi-radial movements needs a full radial restriction from boxes, and the packing in uni-radial movements needs full support and compression from cones. The bi-radial movement subject to the yield and deformation of material needs the packing to be compressed to a fully yielded and deformed state, and so it is possible to exhaust the allowable strength of packing material regardless of whether the working pressure is low or high. The uni-radial movement subject to a shrinking deformation may only need the packing material to deliver a suitable strength in accordance with the sealing requirement or may not need to exhaust the allowable strength for no reason. The grouped square-sectional rings shown in FIG. 3a, with a premade fitting interference, are piece by piece assembled into the box by a little flex deformation, and then fully stuffed into and conformed to the space between the box wall and the stem by stretching and deforming under heavy compression, which needs a full radial restriction from boxes. The V-rings in sets shown in FIG. 3b are fully stuffed into and conformed to the space by mutually-squeezing deformation from their cross-sections under heavy compression, which also needs a full radial restriction from boxes. Actually, all the shaped-rings including V-rings rely on the wedging function of triangles to yield a bi-radial movement as V-rings do, and all need a full radial restriction from boxes. It may be said that all the prior stem cylinder seals are realized not by a fully cone-supported and compressed integral bushing or ring, but by a group or a set of rings mutually-squeezed and deformed in the stuffing box regardless of whether their cross-sections are rectangular or V-shaped or others and whether or not they have a small conical surface used to support and compress them, i.e. all the prior stem cylinder seals are of a boxed seal, a seal with a packing-restricting box, but not a boxless seal. As for the boxed seal:
The first, it is well known that any sealing packing shall be easily deformed under compression; otherwise it can not be used as the sealing stuff filling any unevenness in the jointing surface. So it is beyond any doubt that any V-packing rings and the other shaped packing rings in sets are no rigid body, and will deform to become a packing with an integral-sectional function or to change into a non-wedged body from a wedged body and lose the mechanical characteristic as their original sectional shapes when compressed to some extent, one embodiment of which is that these grouped rings, after compressed to some extent, may not be separated without an externally stripping force, and another embodiment of which is not that the more they are compressed, the higher medium pressure they withstand after compressed to some extent and far before compressed to a broken state.
The second, it is imaginable that any shaped-packing rings in sets will have two opposite deformations at the same time from different partial sections when compressed to deform; one may be a radial increase of partial sections doing sealing work, and the other may be a radial decrease of partial sections doing unsealing work. For example, female Vees always expand to do sealing work, and male Vees always contract to do unsealing work. So any set of packing rings with shaped-sections will always work in such a way that the more they are compressed, the fewer the sealing surface becomes, and the more concentrated the sealing stress becomes to fast exceed the material strength limit and result in a sealing failure, even if they do not yet lose their mechanical characteristics when compressed to some extent. For example, the intermediate V-ring always has a female Vee at its one side, and a male Vee at its other side, with its female Vee expanded to do sealing work and with its male contracted to do unsealing work during being compressed. If Vees do not yet lose their wedging function when compressed to some extent, they may finally have only one external edge circle of female Vees doing sealing work, which both wastes sealing material and has no working reliability. Therefore, any set of packing rings with a shaped-section, particularly for high pressure medium service, should be designed or considered according to some non-wedged bodies or finally regarded as some stuffing material without any wedging function; if not so designed, they will have a worst material-utilizing ratio and a worst working reliability.
The last, it can be seen from the above-analyzed that all the boxed seals are to have their packing fully stuffed into and conformed to the space between the box wall and the stem to finally become one packing with an integral-sectional function and without any interference with its both stuffed box and sealed stem regardless of whether it is in sets or in groups or not, i.e. any packing of all the boxed seals finally relies on transverse strains given by Poisson's ratio to provides radial sealing stresses. The Poisson's ratio of usual sealing material is less than 0.5, such as PTFE with a Poisson's ratio of 0.46, and so the axial strain and stress in the boxed sealing packing are respectively at least 2 times its radial strain and stress; in other words, the maximum load stress in the boxed sealing packing will be at least 2 times its stem-sealing stress after compressed to some extent if the resistance to packing deforming motion is neglected; i.e. the boxed seal may have only a half strength (allowable stress) of sealing material used for stem cylinder seals at most. Thus, to make use of a limited sealing stress or capacity of the boxed sealing packing for a higher medium pressure, the boxed seal has to have a medium-leaking path extended by increasing the axial height of packing, whereas increasing the height is equivalent to decreasing the sealing stress. To keep the sealing stress not changed, it has to have an axial load increased again. To keep the axial stress within the material's allowable stress after increasing the axial load, it has to have an axially force-receiving area increased by increasing the radial dimension of packing again. However, the larger dimension or the more material increased, the more maldistribution of strains and stresses of packing, and the more sealing material wasted. Therefore, the boxed seal is only of one inefficient sealing construction.
Besides, the boxed seal has a stem-embracing force both axially maldistributed because the compressing force is axially progressively decreased, and radially maldistributed because the stem, the packing, the box and the gland can not be in a coaxial or symmetrical assembly, and so when compressed to obtain an integral seal, will have a packing over-compressed to be easy to be worn at some points. That is to say, the boxed seal design has a lower material availability and a lower material wearing resistance. The sealing power is axial to make at first directly the stem shoulder seal operative and then indirectly the stem cylinder seal operative, particularly for the plain packing ring design (see FIG. 3a), and so the stem-sealing actions at the shoulder and the cylinder are out of phase and result in that the sealing gasket at the stem shoulder is often worn or broken when the cylinder seal is operative. Although the V-rings (see FIG. 3b) may provide a radial component with the packing before losing their wedging function as rigid bodies, the air bubble enclosed between male and female V-rings will have a too volumetric change for both the packing seal and the Belleville washer's regulation to be operative in response to temperature and pressure changes, and makes the V-packing ring return back to the same level as the plain ring.
Clause 7.1.1 of ASME B16.34-2004 specifies that valve shells shall withstand a minimum of 1.5 times pressure rating with the valve in the partially open position or including the stem packing, but clause 7.1.3 additionally specifies that leakage through the stem packing shall not be the cause for rejection, and that stem seals shall be capable of retaining pressure at least equal to the 38° C. rating without visible leakage when incapable of withstanding 1.5 times rating. So specify API 6D/ISO 14313 and the other valve standards. That is to say, the prior stem sealing art can not meet the actual requirement so that the valve standards have to make a concession to stem seals by lowering the valve reliability.
In Europe, valve manufacturers have to add one or two O-sealing rings on the stem with the prior packing seal in order to meet the requirement from German TA Luft (Technical Instructions on Air Quality Control).
If the stem-embracing component for stem cylinder seals could be provided by an integral bushing or ring fully supported and compressed between two cones but not by a set of packing rings fully compressed to yield and deform in stuffing boxes, a selection of the two conical angles could adjust both the magnitude of the resultant force radially compressing on the sealing bushing or ring and the matching characteristic of stem cylinder seals and stem shoulder seals. If the radial resultant stress on the sealing bushing or ring could be adjusted to one not less than any other directional stresses, mightn't the stem cylinder seal have all the strength (allowable stress) of sealing material used for the sealing of the stem cylinder and double meet the requirements from ASME standards and TA Luft instructions? If there was a stem shoulder seal matching with such a stem cylinder seal, mightn't the two combined stem seals dually double meet the requirements from ASME standards and TA Luft instructions?